1. Field of the Invention
This invention relates to a method of treating positive photoresist materials applied on a semiconductor wafer and, more particularly, to a method of treating the developed positive photoresist image on a wafer photoresist placed in a chamber employing heating and ultraviolet irradiation.
2. Description of the Prior Art
In the manufacture of semiconductor devices, a process of forming a photoresist pattern comprises the steps of "coating", "prebaking", "exposure", "development" and "postbaking", when roughly divided. Thereafter, for instance, ion implantation or plasma-etching of a silicon oxide film, a silicon nitride film or an aluminum film, which are formed beforehand on the surface of the semiconductor wafer prior to the coating of the photoresist material, is executed. In this case, it is preferable that the photoresist have a high heat-resistance or etching-resistance, since the temperature thereof rises in the process of ion implantation or plasma-etching. However, a photoresist material which has been used for high resolution in recent years for semiconductor devices is highly integrated and made highly fine, is of a positive type photoresist, and the heatresistance of the photoresist of this type is generally inferior to the one of a negative type photoresist.
With a view to enhancing the heat-resistance and plasma-resistance of the photoresist, examinations in a reference of H. Hiraoka and J. Pacansky: J. Vac, Sci. Tech. 19(1981), and U.S. application No. 923,505 discloses a method in which a photoresist material is heated gradually to an elevated temperature for a sufficient time in postbaking, and a method in which ultraviolet radition is applied to a photoresist pattern after development thereof. However, in the former method satisfactory heat-resistance and plasma-resistance fails to be obtained in w short time, that is, the photoresist must be exposed to ultraviolet radiation for a considerably long time. The latter method, on the other hand, has a fault that, although the heat-resistance temperature thereof is raised by exposing the photoresist to ultra-violet radiation, ultraviolet rays do not penetrate to the bottom of the film of photoresist material when it is thicker, which results in an insufficient improvement in the heat-resistance of the entire part in the direction of depth of film of the photoresist material and requires a considerably long exposure time.
In view of these faults, a method of combination of "heating" with "ultraviolet irradiation" has been proposed recently, as is disclosed in U.S. Pat. No. 4,548,688 for instance. This method, however, is unable to meet the demands for improvements in productivity, throughput in a manufacturing process, and etc, since the temperature of the photoresist is raised below the flow temperature of the photoresist driving the entire time of treating the photoresist in a chamber. Thus the faults in the aforesaid prior-art still remain as "problems to be dissolved".
As described above, the prior-art methods for particular treatments of the developed positive photoresist image, such as hardening the photoresist materials employing ultraviolet irradiation, include the problems that a long exposure time for ultraviolet radiation is required for the treatment and that the improvement in the heat-resistance is insufficient in the bottom portion of the developed positive photoresist film when a film is thick, though they have been able to achieve some improvements in the heat-resistance and the plasma-resistance. In other words, there has been left unsettled a problem that the whole treatment of the developed positive photoresist pattern can not be performed in anorganized and effective manner.